The present invention relates generally to apparatus for determining a negative sequence component of a three phase set of alternating current voltages and more particularly to apparatus for detecting phase unbalance or phase loss in a continuous wave frequency transducer.
Several power system control functions require the use of a measurement of frequency of a polyphase set of alternating voltages. Among these are power system stabilizer functions on generator exciters, high voltage direct current transmission systems, and static VAR compensators. These applications benefit from the use of a known continuous wave, phase locked loop type of frequency transducer, which provides wide bandwidth response and less sensitivity to noise. In some of these applications, it is desirable to disable the control function while severe unbalance of the AC system voltages exists or if a failure occurs in any phase of the voltage measuring equipment.
One type of known transducer for sensing frequency in a power system stabilizer (PSS) comprises a continuous wave frequency transducer (CWFT). The CWFT is well known and comprises a phase locked loop circuit which monitors the frequency of the input voltage, i.e. three phase (3.phi.) AC generator terminal voltage, and converts it to a direct current (DC) output voltage whose amplitude is proportional to frequency. The CWFT, in its broadest terms, comprises an implementation of a phase locked loop wherein the three phases of the generator terminal voltage are multiplied by three quadrature phase signal outputs from a voltage controlled oscillator (VCO). The three multiplier outputs are summed, fed to an integrator and a loop filter including a phase lead network, whose output comprises a signal corresponding to actual frequency. This actual frequency signal is compared to a DC reference frequency signal whereupon a frequency error signal is generated which is applied to the voltage controlled oscillator, which changes its output frequency until the error signal goes to zero. The predominate frequency is the fundamental power frequency, i.e. 60 Hz for a 60 Hz system.
Since the CWFT normally operates at zero volts, it is relatively difficult to detect a failure which results in a loss of transmission, since it will continue to operate for a negative sequence, i.e. when one or two of the input phase voltages are lost or a phase shift other than 120.degree. occurs between the three phases.
The present invention utilizes an inherent characteristic of the CWFT to produce a signal proportional to the amount of negative sequence component in the set of alternating voltages. This signal, in turn, may be used for a variety of functions, among them being to switch out, or to adjust the parameters of, critical power system control functions.